Method and apparatus for czochralski growth of large crystals

ABSTRACT

A MODIFIED CZOCHRALSKI METHOD AND APPARATUS FOR SYNTHESIZING AT ATMOSPHERIC PRESSURES LARGE SINGLE MELT-DRAWN CRYSTALS SELECTED FROM THE SULFIDE-SELENIDE-TELLURIDE FAMILY OF THE FOLLOWING PERIODIC TABLE GROUP COMBINATIONS: I-B-V-VI-B, II-B-IV-B-VI-B, II-B-V-VI-B, I-B-VI-B, III-VI-B, IV-B-VI-B, AND V-B, WHICH ARE MELTABLE AT ATMOSPHERIC PRESSURES. A STOICHIOMETRIC MIXTURE IS MADE OF THE DESIRED CRYSTAL CONSTITUENTS, WITH WHICH A SLAG SUPPRESSANT MAY BE ADDED AND PREREACTED. THE STARTING MATERIALS ARE PLACED WITHIN THE CZOCHRALSKI FURNACE CRUCIBLE AND A BOUYANT SOLID NOVEL COVER IS PLACED OVER THESE MATERIALS TO PREVENT VOLATILIZATION OF ONE OR MORE OF THE CRYSTAL CONSTITUENTS, WHICH OTHERWISE WOULD DESTROY STOICHIOMETRY OF THE MELT MATERIALS AND FORM NUCLEATION SITES. A DYNAMIC INERT ATMOSPHERE IS UTILIZED AND THE MATERIALS ARE HEATED INTO A MELT. THE CRYSTALLS ARE THEN DRAWN THROUGH A CENTRAL HOLE IN THE COVER.

June 20, 1972 A GENTlLE I-TIAL 3,671,203

METHOD AND APPARATUS Fori UZOCHRALSKI GROWTH OF LARGE CRYSTALS Filed May 29, 1968 United States Patent O 3,671,203 METHOD AND APPARATUS FOR CZOCHRALSKI GROWTH OF LARGE CRYSTALS Anthony L. Gentile, Thousand Oaks, and Oscar M.

Stafsudd, Los Angeles, Calif., assignors to Hughes Aircraft Company, Culver City, Calif.

Filed May 29, 1968, Ser. No. 733,038 Int. Cl. C01g 3/12, 5/00; B01d 9/00 U.S. Cl. 23-315 19 Claims ABSTRACT OF THE DISCLOSURE A modified C'zochralski method and apparatus for synthesizing at atmospheric pressures large single melt-drawn crystals selected from the sulfide-selenide-telluride family of the following Periodic Table Group combinations: I-B-V-VI-B, II-B-IV-B-VI-B, II-B-V--VI-B, I-B-VI-B, HI-VI-B, IV-B-VI-B, and V-VI-B, which are meltable at atmospheric pressures. A stoichiometric mixture is made of the desired crystal constituents, with which a slag suppressant may be added and prereacted. The starting materials are placed Within the Czochralski furnace crucible and a buoyant solid novel cover is placed over these materials to prevent volatilization of one or more of the crystal constituents, which otherwise would destroy stoichiometry of the melt materials and form nucleation sites. A dynamic inert atmosphere is utilized and the materials are heated into a melt. The crystals are then drawn through a central hole in the cover.

The present invention relates to a modified Czochralsk method and apparatus for synthesizing large single meltdrawn crystals selected from the sulfide-selenide-telluride family. Such crystals are compounded from the elements selected from the Groups of the Periodic Table of the Elements, in accordance with the periodic classifications of Glasstone The Elements of Physical Chemistry, page 19, D. Van Nostrand Company, Inc., 1946, and of Moeller Inorganic Chemistry, pages 122, 125, 126, John Wiley & Sons, Inc., 1952, consisting of the following combinations: I-B-V-VI-B, Il-B-IV-B-VI-B,

II-B--V-VI-B I-B--VI-B, III-VI-B, IVAB-VI-B, and V-VI-B, which are meltable at atmospheric pressures. Therefore, as used herein, the various crystals synthesized by the inventive process are termed crystals of the type described or crystals of the class described, the type or class lbeing selected from the above listed Periodic Table groupings.

The modified Czochralski apparatus used to obtain these crystals comprises a Czochralski furnace utilizing novel solid and buoyant cover through which the crystal is drawn.

The synthesis of stoichiometrically pure proustite (AgaAsSa), pyrargyrite (Ag3SbS3), tapalptite (Ag3BiS3) and numerous other compounded crystals of the class described have been heretofore difficult, in part, because these materials, when heated to form a melt, decompose under atmospheric pressures and undesirably react to form slag or dross. This slag then acts as a nucleation center wherever it comes in contact with the solid-liquid interface of the growing crystal and creates a non-stoichiometric crystal. This condition is caused by the evaporation from the heated melt of the individual constituents at different rates. The less volatile constituents remain in solution and unbalance the stoichiometry of the melt. These remaining constituents also react with each other to form slag which rises to the surface of the melt. The growing crystal,

3,671,203 Patented June 20, 1972 ice therefore, is drawn from the melt not having the desired stoichiometric proportions of the crystal materials. At the same time during the drawing operation, the slag causes spurious crystal growth and adheres to the growing crystal, m1l1tating against a chemically pure crystal.

In an effort to overcome these problems, the prior art has utilized a liquid encapsulation technique by which a molten cover is placed over the melt in order to control evaporation. A hole is then pierced through the liquid encapsulation by the crystal drawing rod for contact with the melt. This technique still does not solve the slag forming problem and additionally requires a liquid whose density must be lower than that of the melt so that it will float thereon. In addition, liquid encapsulation does not afford adequate thermal insulation for the melt from the cooling effect of an inert atmosphere, thereby producing a thermal gradient in the melt and making control of the process extremely diicult. For low temperature melts, the liquid encapsulation becomes so viscous that it is not possible to properly draw a crystal therethrough. Other materials, having the desired fluidity at low temperatures, such as the halides, react with the melt causing further problems in obtaining a pure crystal.

Other techniques include the utilization of a pressurized. furnace; however, pressurization necessitates a very expensive furnace and requires the use of very small ports through which the process could be viewed. These pressurizaton techniques nevertheless do not adequately overcome the problems of evaporation and the formation of slag.

The present invention overcomes these and other problems by providing a melt of the desired crystal constituents which are `mixed together in stoichiometric proportions. A high chemical purity of materials is required. When, because of decomposition problems, further control is required, a slag suppressant is further mixed with the starting materials. It has been found that a suppressant is required for such crystals as proustite, pyrargyrite, and tapalptite. In other materials, such as lead-tin-telluride (Pb1 xSnxte) and synthetic zinkenite (PbSbZSQ the use of slag suppressant is not necessary.

The first group of the proustite family includes those elements selected from the Periodic Table of the Elements consisting of the Groups I-B-V-B-VI-B and the suppressant comprises those elements selected from the Periodic Table consisting of the V-B-VI-B Groups. For example, in proustite, the suppressant comprises arsenic trisulfide. In pyrargyrite the suppressant comprises antimony trisulde. In tapalptite the suppressant comprises bismuth trisulide. Because these materials do not react satisfactorily at the temperatures used within the Czochralski draw furnace, it is necessary to first combine them in a furnace at an elevated temperature. The starting materials are placed within an evacuated fused silicon ampoule and reacted at approximately 900 C. Thereafter, the mixture is quenched and removed from the ampoule and placed within a crucible within the Czochralski furnace.

With other materials, such as lead-tin-telluride and synthetic zinkenite, no suppressant is required and these materials can be placed directly within the crucible of the Czochralski furnace.

After placement of the starting or charge materials within the Czochralski furnace crucible, a solid cover is placed over these materials. The cover is designed to fit closely within the crucible so that there is as little space as possible between the covers outer periphery and the crucibles inner periphery. In addition, the cover is provided with a central hole through which the crystal may be drawn. The cover may be of a density which is lower than that of the melt to be prepared from the charge materials, or, if desired the cover may be boat-shaped if its density is greater than that of the melt in order to provide flotation compartments in the cover.

The underside of the cover which contacts the melt may be at or, if desired, it may be undercut to provide an annular recess between the opening in the cover and the periphery thereof. It has been found that, when slag or dross forms on the melt, formation is at the portions of the melt adjacent to the inner walls of the crucible and thereafter the slag oats inwardly toward the center of the melt. Since the present invention affords very close control of the amounts of slag produced, the cover effectively precludes flotation of slag; however, the recess in the underside of the cover may be used to further limit movement of slag toward the region where the crystal is grown by providing a mechanical stop.

In addition, the central opening in the cover may be beveled in such a manner that it opens outwardly from the melt. This beveled configuration permits a crystal to be grown whose diameter is larger than that of the smallest portion of the beveled opening. It is believed that this configuration acts as a wick whereby the melt ows upwardly along the sides of the opening, thereby creating the larger diameter crystal.

The cover is made of a material which is non-reactive with the melt and preferably is formed of boron nitride, although graphite, aluminum oxide, magnesium oxide and fused silica` operate satisfactorily. Boron nitride is preferable because it is not wetted by the melt.

After the mixture of starting materials of the crystal have been placed in the Crucible and the cover is placed over the mixture within the Czochralski furnace, the environment is sealed and an inert gas, such as nitrogen or helium, is flowed Within the sealed environment at atmospheric pressure. A pulling rod having a seed at one end thereof is placed within the furnace and is extended outwardly through a seal to a drawing and rotating mechanism. The rod is formed of any inert material such as fused silica, graphite, platinum, iridium, and a platinumiridium alloy and is used as a cold nger heat sink to begin crystal growth. If desired, the tip of the rod may be etched to enhance adhesion of the crystal to be drawn from the melt.

The mixture is then heated by any convenient means, such as by radio frequency (RF) or resistance coils, until the mixture liquilies as a melt. The rod is then lowered through the cover opening suiciently into the melt so that the seed or the tip of the rod contacts the melt. The rod is rotated at a rate suicient to prevent the formation of radial thermal gradients at the portion of the melt from which the crystal will be drawn. The rod is then raised at a rate commensurate with the growth of a crystal and, after the desired length of crystal has been grown, the process is stopped and the crystal is removed from the furnace. Because of the novel solid cover, the rate at which the rod is raised is not critical.

If the heating source is an RF coil, a water jacket may be placed about the sealed enclosure so as to cool the enclosure and to maintain the seal thereof.

It is, therefore, an object of the invention to provide a method for growing crystals of the type described.

Another object is the provision of a novel melt encapsulation cover for use in a Czochralski draw furnace.

Other aims and objects, as well as a more complete understanding of the present invention, will appear from the following explanation of exemplary embodiments and the accompanying drawings thereof, in which:

FIG. 1 is a diagrammatic view of a Czochralski furnace with its temperature profile and further illustrating a cover used therefore; and

FIGS. 2-6 are various embodiments of the cover used in the furnace, with FIG. 5 being a section taken along lines S-S- of FIG. 4.

Accordingly, a Czochralski draw furnace comprises a sealed enclosure 12 preferably formed of fused silica for '4 enclosure of a crystal growing apparatus 14. Enclosure 12 is provided with a gas inlet 16 and an outlet 18 so that an inert gas atmosphere may be provided within the enclosure.

Crystal growing apparatus 14 comprises a Crucible 20 having an upstanding wall 22 to form a receptacle 24 for reception of a mixture of crystal ingredients to be heated into a melt 26. A platform 27 supports the crucible. A cover 28 of solid material is placed above melt 26 and floats thereon. Crucible 20 and cover 218 are fabricated of materials which are nonreactive with the melt. Both may be made of a compacted graphite; however, cover 28 is preferably made of boron nitride although other materials such as fused silica, aluminum oxide, and magnesium oxide are also suitable. Boron nitride is preferred because it has the ability not to be wetted by the melt.

Cover 28 is provided with an outer periphery 30 which closely fits within the inner periphery 32 of the crucible. A central opening or hole 34 is formed in cover 28 so that a crystal 36 may be drawn from the melt. The crystal is drawn by means of a rod 38 which extends from enclosure 12 through a sealed opening 40 for connection to a rotating and drawing mechanism. The rod may be supported by standards 42 and a plate 44 in any convenient manner.

A plurality of coils 46 are disposed [about crucible 20 in order to heat the same and these coils may be designed to be operated by an RF source or by resistance heating. If the heating is by an RF means, crucible 20 must be made of or surrounded by a material which is a suitable RF susceptor in order to be heated by the RF coils. If the heating means comprises a resistance source, then any suitable non-reactive crucble material may be utilized.

When an RF source is used, the source is enclosed within a jacket 48 in which water or other coolant 50 is placed in order to maintain enclosure 12 cool and sealed and to maintain the desired inert atmosphere in the furnace.

The furnace is provided with a temperature profile 52 having an isothermal portion 54 and a decreasing temperature gradient 56. A point 58 on gradient 56 is the point at which melt 26 becomes a solid, this point designating the solid-liquid interface between the melt and the crystal. Point 58 of temperature profile 52 is controlled as to be located at cover 28 so that crystal 36 may be drawn from melt 26.

Cover 28 may be provided with several configurations, as shown by FIGS. 2-6. In general, the cover has an upper face 60 and a lower face 62, the upper face extending away from melt 26 while lower face 62 is in contact therewith.

Cover 28 is shown in its simplest form in FIG. 2 as a disc having a parallel sided opening 34a therein. In FIGS. 3 and 4, opening 34b is beveled, having the appearance of a truncated right circular cone, and widens from face 62 to face 60 to provide a narrow diameter 64 and a wide diameter 66. This beveled surface causes a wicking action of the melt whereby the diameter of crystal 36 is greater than diameter 64 of opening 3419 at face 62.

In FIG. 4, face 62 may be undercut to provide a recess 68 of, for example, annular shape in order to provide a means for preventing slag or dross from reaching recess 34.

In FIGS. 2-5, cover 28 is made of a material whose density is less than that of melt 26 so that it will float on the surface of the melt. However, if cover 29 should be of higher density than the melt, then the embodiment of FIG. 6 may be utilized. In this embodiment, cover 28 has a boat-like conguration to provide buoyancy compartment 70. Therefore, the embodiment of FIG. 6 will oat upon the surface of the melt and, if desired, the cover may also be provided with a recess 68.

As stated above, the present invention is useful for synthesizing crystals selected from the sulfide-selenidetelluride family except for those compounds which are not meltable at atmospheric pressure. Such crystals include the Ml-Mz--VI-B and Ml--VI-B type compounds. In the Ml-Mz-VI-B compounds, Ml-MZ can be of any of the following Groups of the Periodic Table: I-B-V, II-B--IV-B, and II-B-V, wherein the Group V elements may be either A or B of the Periodic Table. In the Ml-VI-B type compounds, the groups represented by M1 are the following Groups of the Periodic Table: II-B, IV-B, III-A and III-B, and V-A and V-B. A brief table of such combination of elements, identified by Group, according to the periodic classifications of Glasstone and Moeller, noted above, is set forth below for convenience.

Group Mi-VI-B compounds Group Mi-Mzf-VI-B compounds I-B-VL-B I-'B-V-A-VI-B I-B-V-B-VI-B II-B-IV-B-VI-B III-A-VI-B III--VI-B II-B-V-A-VI-B II-B-V-B-VI-B The example as set forth below relates to the synthesis of proustite (Ag3AsS3); however, all the crystals of the class described may be made in a manner similar to the following example any differences vbeing noted in a table following this example, for several other illustrative crystals.

EXAMPLE The mixture was placed in a fused silica ampoule which was then evacuated and sealed at a pressure of -E torr. However, if desired, an inert gas is as suitable since it is only necessary to avoid contamination and excessive pressures within the ampoule. The ampoule was then placed in a clam shell furnace and heated to approximately 1,000 C.; however, any furnace which would provide the temperature of 1,000 C. may be substituted for the clam shell furnace. The mixture was left in the furnace until the constituents chemically reacted. This reaction occurred in approximately one hour. The reacted mixture was then cooled and removed from the ampoule.

This reacted mixture was then placed in a crucible, such as crucible 20, in a Czochralski furnace. A cover, such as cover 28, was placed over the reacted mixture, draw rod 38 was placed in position over the Crucible and attached to a rotating and drawing mechanism, and enclosure 12 was sealed. A dynamic inert atmosphere at atmospheric pressure was created within enclosure 12 by means of the ilow of nitrogen gas from inlet 16 to outlet 18. Coils 46 were then energized by an RF source and the mixture was observed until it became a melt, such as illustrated by melt 26. At the saine time, a coolant 50 was circulated over the exterior of enclosure 12 to maintain and preserve its seal. The mixture became a melt at approximately 500 C., which was noted by observation.

Rod 38, with a seed at its end, was then lowered through opening 34 of cover 28 into contact with the melt. At the same time, rod 38 was rotated at a speed of approximately 30 r.p.m. The rod was drawn upwardly at a rate of approximately 1 centimeter per hour. After the desired size of crystal had been formed, the RF source was turned off, the seal of enclosure 12 broken, and a crystal 36 of proustite was removed.

Proustite, as well as other materials, are listed below in the following table to illustrate the types of crystals which may be grown by the inventive method.

Tempera- *This measurement was not accurately taken since the point at which the melt formed was noted by visual observation.

Although the invention has been described with reference to particular embodiments thereof, it should be realized that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

What is claimed is: 1. A method for growing crystals from the class selected from the Groups of the Periodic Table of the Elements consisting of I-B-V-VI-B, II-B-1V-B VI-B, AII-B--V--VI-B, IB-VI-B, ILI-VI-B, lV-B- VI-B, and V--VI-B, and meltable at atmospheric pressures from constituents of the crystals within a Czochraldraw furnace provided with a temperature gradient having a crystal liquid-solid interface point comprising the steps of combining the constituents of the crystals in stoichiometric proportions to provide a mixture of combined constituents, u

placing the combined constituents into a crucible in the furnace,

placing a solid cover having central opening means over the combined constituents, the solid cover having a close iit within the crucible and tloatable 0n a melt of the combined constituents,

heating the combined constituents to form the melt and floating the solid cover on the melt to prevent volatilization of the melt, to maintain the stoichiometric proportions, and to inhibit formation of spurious nucleation sites, and

drawing at the interface point a crystal from the melt through the opening means.

2. A method as in claim 1 wherein said class is selected from said Groups consisting of I-B-V--VI-B, II-B-V-VI-B, and V-VI-B and further comprising the step of including a dross suppressant with the combined constituents, the dross suppressant selected from the class of compounds consisting of the V-V'I-B elements of the combined constituents.

3. A method for growing a crystal of the class selected from the Groups of the Periodic Table of the Elements consisting of I-B-V-VI-B, II-B-1VBVIB, II-B-V-VI-B, I-B-VI-B, III-VIAB, IV-B-VI-B, and V-VI-B and meltable at atmospheric pressures within a draw furnace provided with a temperature gradient having a crystal liquid-solid interface point comprising the steps of preparing a melt of constituents of the crystal within the furnace;

floating a solid cover on the melt at the interface portion, the cover having opening means therein to inhibit volatilization of the melt and formation of spurious nucleation sites; and

pulling the crystal from the melt through the opening means.

4. A method as in claim 3 further including the step of utilizing a dynamic inert atmosphere within the furnace at atmospheric pressure.

5. A method as in claim 3 further including the steps of combining 'said crystal constituents in stoichiometric proportions before proceeding to said melt preparing step, said floating cover maintaining the stoichiometric proportions.

6. A method as in claim wherein said combining step further includes the step of reacting said crystal constituents in a heated evacuated and enclosed environment.

7. A method as in claim 5 wherein said class is selected from said Groups consisting of I-V-V-VI-B, II-B- V-VI-B, and V-VI-B and wherein said combining step further includes the step of adding a dross suppressant to said constituents, the dross suppressant selected from the class of compounds consisting of the V--VI-B elements of the crystal constituents.

8. A method for fabricating a crystal comprising the steps of utilizing a mixture containing stoichiometric proportions of elements selected from a irst group consisting of the I-B-V-VI-B Groups of the Periodic Table of Elements and containing elements selected from a second group consisting of the V-VI-B Groups of the Periodic Table of Elements, said elements of said second group being among the same said elements of said first group, said elements being mixed in the ratio of 95 parts by weight of said rst group elements to 5 parts by weight of said second group elements,

sealing said mixture in a fused silica ampoule,

heating said sealed mixture in a furnace at a temperature suicient to cause a reaction among said elements,

quenching and removing said reacted mixture from said ampoule, placing said reacted mixture in a crucible in a sealed Czochralski furnace,

placing a solid cover over said reacted mixture within the crucible, said cover having a close peripheral lit with the sides of the crucible and having central opening means, and said cover further oatable on a melt of the reacted mixture,

heating said reacted mixture to produce the melt and oating said cover on the melt to inhibit volatilization of the melt and formation of spurious nucleation sites and to maintain the stoichiometric proportions, and

pulling the crystal from the melt at a rate commensurate with crystal growth.

9. A method as in claim 8 wherein said crystal pulling step includes the steps of utilizing a crystal pulling apparatus, and

rotating the apparatus at a rate suflicient to prevent radial thermal gradients in said melt.

10. A method as in claim 8 wherein said Group I-B elements of said first group are selected from the elements consisting of copper, silver, and gold.

11. A method as in claim 8 wherein said Group V elements of said first and second groups are selected from the elements consisting of arsenic, antimony and bismuth.

12. A method as in claim 8 wherein said Group VI-B elements of said lirst and second groups are selected from the elements consisting of sulfur, selenium, and tellurium.

13 In the use of a Czochralski method of growing compound crystals from a melt comprising at least two elemental constituents and utilizing a Czochralski draw furnace having a draw rod and a crucible having an inner periphery, the improvement for inhibiting stoichiometric imbalance of the melt through volatilization of at least one of the constituents and decomposition of the melt and for inhibiting the formation of spurious crystal nucleation centers comprising: a solid cover floatable on the melt and having central opening means for reception of the draw rod and further having an outer periphery of substantially the same dimensions as those of the inner periphery of the crucible to substantially seal the melt within the crucible at the inner periphery thereof and to inhibit the volatilization and the decomposition of the melt.

14. The improvement as in claim 13 wherein said cover further includes at least one face for contact with the melt and under-cut means in said cover at the one face positioned between said opening means and the outer periphery.

15. The improvement as in claim 13 wherein said cover is of higher density than the melt and further including a pair of oppositely placed faces on said cover, a first of the faces being disposed for contact with the melt and a second of the faces being disposed for positioning away from the melt, and buoyancy compartment means in said cover at ythe second face and positioned between said opening means and the outer periphery to provide said cover with a boat-like conguration whereby said cover is oatable on the melt.

l16. The improvement as in claim 13 wherein said central opening means comprises a wicking means.

17. The improvement as in claim 13 wherein said central opening means comprises a beveled opening means widening from one face of said cover to the other face of said cover.

18. The improvement as in claim 13 wherein said cover comprises a material which is non-reactive with the melt.

19. The improvement as in claim 13` wherein said cover is selected from chemical materials consisting of boron nitride, fused silica, aluminum oxide, magnesium oxide, and graphite.

References Cited UNITED STATES PATENTS 2,686,212 8/1954 Horn et al. 14S-1.6 3,002,824 10/1961 Francois 148----1.6` 3,394,994 7/1968 Faust, Jr. et al. 148--1.6 3,527,574 9/ 1970 La Belle, Jr. 23-273 SP OTHER REFERENCES Liquid Encapsulation Techniques: The Use of an Inert Liquid in Supressing Dissociation During the Melt- Growth of InAs and GaAs Crystals, by Mullin et al., Journal of Physics and Chemistry of Solids, vol. 26, pp. 782-784.

A Technique for Pulling Single Crystals of Volatile Materials, by Metz et al., Journal of Applied Physics, vol. 33, No. 6, pp. 2016-2017, June 1962.

OSCAR R. VERTIZ, Primary Examiner H. S. MILLER, Assistant Examiner U.S. C1. X.R.

23-50, 134, 273 SP, 277 R 

